The present invention relates to an epoxy resin-based curable composition capable of giving cured products having excellent mechanical and electric properties and resistance against water as well as remarkably improved resistance against formation of cracks with a glass transition temperature not lower than in conventional epoxy resin-based cured products. In particular, the invention relates to and provides a basic formulation of an epoxy resin-based curable composition capable of giving a cured product having remarkably improved resistance against formation of cracks with low internal stress as are the essential requirements in the resin compositions suitable for packaging or encapsulation of electric or electronic parts and devices.
As is well known, a variety of resin compositions are currently used for the plastic packaging or encapsulation of electric or electronic parts and devices including the compositions formulated with the base resins of, for example, thermosetting resins such as epoxy resins, silicone resins, polybutadienes, polyurethanes, phenolic resins and the like as well as various thermoplastic resins utilizing their respective advantageous properties according to the desired applications.
Among the above named resin compositions, those based on an epoxy resin are used most widely and in the largest quantities by virtue of their excellent mechanical and electric properties, heat resistance and adhesion as well as their good workability or moldability. In particular, epoxy resin-based curable compositions predominantly occupy an outstanding position by virtue of the excellent performance thereof as a resin composition for encapsulation of semiconductor devices such as diodes, transistors, ICs, LSIs and the like under rapid technical growth in recent years.
Even though the epoxy resin-based curable compositions have been hitherto quite satisfactory in almost all respects as an encapsulating resin composition, there is a growing demand for an epoxy resin-based curable composition having more and more improved performance to comply with the development of the technology of electronics or, in particular, the trend of thinner and thinner or smaller and smaller design of the electronic devices and the increase in the density of integration as in LSIs. Accordingly, the conventional epoxy resin-based curable compositions are already not always quite satisfactory materials to meet such high-grade requirements in the modern electronics technology.
That is, improvements are desired for epoxy resin-based curable compositions in several aspects including, for example, higher and higher purity to prevent contamination of the semiconductor devices, higher electric performance as a matter of course, improved moldability to ensure shortened molding cycles contributing to the increased productivity, higher heat conduction or heat dissipation to ensure applicability to high-power devices, lower stress in the cured product to protect the encapsulated electronic device from an excessive physical stress and increased resistance against formation of cracks to withstand any severe thermal and mechanical shocks. Needless to say, many attempts have been undertaken to obtain epoxy resin-based curable compositions improved in these respects but particular difficulties are encountered in obtaining compatibility between the higher heat conductivity and the improved resistance against crack formation or decreased stress without adversely affecting the other characteristics so that no epoxy resin-based curable compositions have not yet been developed as imparted with the above described improved properties in combination to be a promising encapsulating resin composition in modern electronics industry.
Most of the epoxy resin-based curable compositions currently on use for the encapsulation of semiconductor devices are formulated with a bisphenol-type epoxy resin or a novolactype epoxy resin as the base component filled with a large volume of a crystalline or amorphous silica filler together with a crosslinking or curing agent to effect crosslinking of the polymer molecules with heating.
Although it is a relatively easy matter to satisfy either one of the above described requirements alone, it is sometimes rather a formidable problem to obtain improvement of the performance of the resin compositions in one particular point without sacrifice of one or more of the other properties and the requirements for different properties are sometimes incompatible with each other so that mere extension of hitherto undertaken way of investigations is of power no more.
For example, it is a conventional measure hitherto undertaken when improvements in the decreased stress and increased resistance against crack formation are desired to formulate the epoxy resin-based composition with a flexibility-improver such as 1,4-butanediol, polyoxyalkylene ether glycol, glycerin, polysulfide polymer, polyoxyalkylene glycidyl ether and the like but formulation of these additives is always accompanied by lowering of the glass transition temperature and decrease in the heat resistance as well as resistance against moisture or water. On the other hand, improvement in the heat conductivity is readily achieved by formulating the resin composition with a large volume of powdery crystalline silica alone as the filler though with unavoidable problems of decreased resistance against crack formation and larger stress or a coefficient of thermal expansion. In short, extreme difficulties are encountered in solving the above mentioned problem of obtaining compatibility in the improvements in respects of the high heat conductivity and resistance against crack formation as the largest requirements for the epoxy resin-based curable compositions insofar as the way of investigations is limited on the conventional route.